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Credit Facilities, Long-Term Debt and Lease Liabilities - Credit Facilities (Details) - CAD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2019 | Dec. 31, 2020 | Dec. 31, 2018 | Dec. 31, 2017
Disclosure of detailed information about borrowings [line items] |  |  |  |
Cash and cash equivalents | $ 411 | $ 703 | $ 89 | $ 314
Credit facilities | Committed credit facilities |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |
Undrawn borrowing capacity under line of credit | 1,300 | 1,500 |  |
Corporation and TransAlta Renewables | Syndicated Bank Credit Facility |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |
Maximum borrowing capacity under line of credit | 1,950 | 1,950 |  |
TransAlta Renewables Inc. | Syndicated Bank Credit Facility |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |
Maximum borrowing capacity under line of credit | 700 |  |  |
Line of credit facility, increase (decrease) | 200 |  |  |
TransAlta OCP |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |
Restricted cash (Note 22) | $ 17 | 17 |  |
Restricted cash and cash equivalents, principal amount |  | $ 11 |  |

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